The output spectrum of a line-narrowed excimer laser light source for DUV lithography is not generally constant in time. While stability has greatly improved with advances in technology, neither the bandwidth nor the functional form (shape) of the spectrum is perfectly fixed. The impact of spectral shape changes on lithographic performance has so far been not completely characterized, however, the influence of full-width at half-maximum (“FWHM”) and 95%-enclosed energy (“I95%” or “E95” or sometimes referred to as “spectral purity”) illumination bandwidths on image contrast, log-slope, exposure latitude, etc., have been found to be significant, as discussed in “Contribution of polychromatic illumination to optical proximity effects in the context of Deep-UV lithography”, A. Kroyan, I. Lalovic, N. R. Farrar, Proc. 21st Annual BACUS Symposium on Photomask Technology and Management, G. T. Dao and B. J. Grenon (Eds), Monterey Calif., SPIE Vol. 4562, pp. 1112-1120, 2002 and “Understanding chromatic aberration impacts on lithographic imaging”, K. Lai, I. Lalovic, R. Fair, A. Kroyan, C. Progler, N. R. Farrar, D. Ames, K. Ahmed, J. Microlithography, Microfabrication and Microsystems, Vol. 2, Issue 2, pp. 105-111, 2003, the disclosures of which are hereby incorporated by reference.
Dependence on the illuminating spectrum arises, e.g., because optical material constraints at DUV wavelengths make some chromatic aberration unavoidable in projection lenses for KrF and ArF lithography. While chromatic effects can be minimized with a spectrally narrowed light source, even sub-picometer broadening of the illumination spectrum cannot be completely ignored, as discussed in “Modeling the effects of excimer laser bandwidth on lithographic performance” A. Kroyan, J. J. Bendik, O. Semprez, N. R. Farrar, C. G. Rowan and C. A. Mack, SPIE Vol. 4000, Optical Microlithography XIII, pp. 658-664, March 2000, the disclosure of which is hereby incorporated by reference. The concern becomes even more pressing as the industry moves to ever-higher numerical aperture settings and lower values of k1. To guarantee that the aerial image properties are maintained within a given process window, it is therefore increasingly more important to have trustworthy metrologic feedback from the light source reporting these spectral figures-of-merit with high accuracy and reliability and stability. Further, in more advanced applications this information can actually be used to control the workings of the light source in some way, so as to stabilize the light source spectrum or otherwise modulate its bandwidth. In such scenarios, the enhanced spectral performance repeatability obtained means that generic optical-proximity (OPC) solutions can be imagined that remain effective and consistent over the system lifetime, e.g., including requirements for enhanced ability to strictly control bandwidth within some range, i.e., below some threshold but also above some threshold.
Commonly used bandwidth metrics such as FWHM and E95 are not always accurate measures of spectral shape, especially when either is considered alone. For example, an increase in the energy content of the far wings of a spectrum can significantly increase the E95 bandwidth value, while leaving the FWHM bandwidth value essentially and effectively unchanged. Other spectral shape changes can, e.g., leave the E95 constant while altering the FWHM, or can, e.g., leave both these metrics constant while changing the center-of-energy of the spectrum or other performance-significant parameter of the spectrum. These shape changes can often go hand-in-hand with, e.g., bandwidth changes, with significant consequences for the design of spectral metrology tools and the performance of systems relying upon their effectiveness in accurate bandwidth estimation, and particularly in systems, which are becoming ever more prevalent, where metrology feedback and concomitant control functions are required to be on a pulse by pulse basis at repetition rates to and exceeding 4000 Hz.
Variations in the detailed shape and bandwidth of ultra-narrow excimer laser light sources can originate in a variety of physical mechanisms. Some of this variation is technically unavoidable, and a somewhat effective strategy to overcome this in the past has been to design the light source in a manner that is generally optimized to minimize the effects of such variation. Even with engineering controls, however, large changes in spectral shape or bandwidth can sometimes occur due to improper alignment, failure of optical components, or failure to manage important process parameters internal to the light source (e.g., laser gas mixture). It is the job of the onboard spectral metrology package to correctly identify and accurately report the light source bandwidth so that it may be used as trustworthy input to the lithographic process control. To illustrate these shape changes, a number of examples of typical spectral shape variation seen in a Cymer XLA 100 ArF MOPA (Master-Oscillator/Power-Amplifier) light source measured with a high-resolution double-pass echelle grating spectrometer are shown in FIGS. 1A-D. This collection is not exhaustive, but is believed to be typical of a light source of the current generation. The data has been normalized to equal total energy content for a better comparison of the spectral energy distribution, and to better represent the integrated spectral content for an exposure of, e.g., 200 laser pulses.